Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Light has two effects on the melatonin rhythm: acute suppression and entrainment (phase shifts) of the underlying clock. Vitamin A depletion and retinaldehyde addition markedly affected the acute response to light without affecting the size of phase shifts. Separate pathways and photopigments mediate the two effects of light. Exposing cells to increased temperature (40- 43degreesC) increased melatonin output, induced heat shock proteins, and caused light-like phase shifts.